WO2002103695A2 - Device and method for embedding a watermark in an audio signal - Google Patents

Abstract

The invention relates to a method for embedding a watermark in an audio signal, according to which a spectral representation of the audio signal and a spectral representation of the watermark signal are first determined (14, 16). The spectral representation of the watermark signal is then processed, based on a psychoacoustic masking threshold (24) of the audio signal (22). The processed watermark signal is combined with the audio signal (18). The spectral representation of the watermark signal is processed iteratively in the following way: a predetermined watermark initial value (26) is first selected; the interference that has been introduced into the spectral representation by the predetermined watermark initial value after a quantization of the spectral representation of the audio signal, is determined (28). The watermark initial value is then modified until said interference introduced into the spectral representation of the audio signal by a modified watermark initial value after quantization, is less than or equal to the predetermined interference threshold (32).

Description

 Device and method for embedding a watermark in an audio signal

description

The present invention relates to the field of audio coding and in particular to methods and devices for embedding a watermark in an audio signal.

Modern audio coding methods process discrete-time audio samples to provide a bit stream that is compressed relative to the original audio signal. The stream of discrete-time audio samples is first windowed to generate successive blocks of windowed audio samples from the stream of audio samples. Further processing takes place in blocks. A block of audio samples generated by windowing is typically converted into a spectral representation by means of an analysis filter bank. In terms of frequency, the spectral representation comprises spectral values lying next to one another from frequency 0 to the maximum audio frequency, which can be, for example, 16 kHz. The audio spectral values are grouped and quantized in scale factor bands. The quantization takes place in such a way that the quantization noise introduced by the quantization is dimensioned such that it is masked by the audio signal. For this purpose, a psychoacoustic model is used which, on the basis of the audio signal, supplies an energy value for each scale factor band, which indicates the energy level up to which quantization noise is masked, ie it will not be audible in the decoded audio signal. If, on the other hand, the quantization noise introduced by the quantizer lies above the psychoacoustic masking threshold, the re-decoded audio signal will contain audible interference. The quantizer levels of the quantizer are dependent on the masking threshold calculated. When the quantization levels are calculated, the audio spectral values are quantized using these quantization levels to obtain quantized audio spectral values. For data efficiency reasons, the quantized audio spectral values of an entropy coding, such as B. Huffman coding, to provide a bit stream with code words that represent the audio spectral values. Using a bitstream multiplexer, page information is added to the stream of code words, which includes, among other things, the scale factors on the basis of which an audio decoder can determine the quantization levels that have been used in the encoder.

For audio decoding, the bit stream including side information is split up into code words on the one hand and side information on the other using a bit stream demultiplexer. The entropy coding is first undone. Then the entropy decoded, values quantized audio spectral values, an inverse quantization that is subjected to inversely quantized spectral values ^¬ to get. These are then converted from the frequency domain to the time domain using a synthesis filter bank. The decoded audio signal is present at the output of the synthesis filter bank.

It should be pointed out that this is a lossy coding method since quantization has been carried out in the encoder. The decoded audio signal does not exactly match the original audio signal. If the coding and decoding were successful, however, the subjective auditory impression of the decoded audio signal will correspond to the subjective auditory impression of the original audio signal, since the quantization noise introduced by the quantizer in the encoder is masked away, ie it is "hidden" below the psychoacoustic masking threshold. For data efficiency reasons, it is preferred to use the largest possible quantization steps. On the other hand, excessively large quantization steps lead to excessive quantization noise, which can manifest itself as an audible disturbance in the decoded signal. Modern audio coding methods try to achieve an optimal compromise between these two requirements.

The psychoacoustic masking threshold of an audio signal section depends on the actual input audio signal. If the audio signal changes over time, the masking properties also change over time. For reasons of data efficiency, it is preferred to always introduce as much quantization noise into the audio signal as possible, ie the quantization noise should correspond as closely as possible to the psychoacoustic masking threshold. Audio signal sections with good masking properties can therefore be encoded with a relatively low bit cost, while on the other hand audio signal sections with relatively poor masking properties, such as. B. tonal audio signal sections, must be quantized very finely, which in turn means that a large number of bits must be used to encode these audio signal sections. An encoder that tries to always introduce exactly the amount of interference given by the masking threshold will therefore produce a constant quality audio signal. However, due to the time-varying nature of the input signal, this leads to a variable bit rate at the output of the encoder. Although coding with constant quality - and thus with a variable bit rate - is attractive for data efficiency reasons on the one hand and audio quality reasons on the other hand, this concept is disadvantageous in that it is only suitable for applications which support a variable transmission rate, such as z. B. the storage of compressed audio signals or the transmission of compressed audio signals over packet-based networks, such as. B. the Internet. However, many applications require an audio encoder with a constant transmission rate. Due to the temporally varying spectral and temporal properties of an audio signal, this inevitably leads to variable quality. In particular, depending on the bit rate, the case may arise in which sections of the audio signal which have relatively low masking properties cannot be quantized sufficiently finely, ie are under-coded and may contain audible interference in the decoded signal, while on the other hand segments which are easy to code, ie audio signal section with good masking properties, must be encoded more precisely than necessary, ie over-encoded.

In order to overcome the disadvantages of over-coding and under-coding, a bit savings bank function is usually used. The bit savings bank is filled when easy-to-encode audio sections are encoded such that bits that are not needed to encode these easy-to-encode sections are not simply "wasted" by more subtle than necessary quantization, but nevertheless a coarser quantization is used and the surplus bits are "inserted" into the bit savings bank.

Come contrast, audio segments before that are difficult to encode, ie where lower Quantisiererschritt- must be wide used than is actually possible by the required constant average data rate, it is to this end, the bit reservoir "emptied" to calling despite the ge ^¬ use data rate, a finer quantization than Eigent ^¬ Lich possible, so that contained in the decoded audio signal in these sections no audible interference. The bit savings bank function thus acts as a buffer,

to make an audio encoder with constant bit rate to the "outside". Nowadays, music distribution via the Internet, for example, is becoming an increasingly important technology. Most of the music content is compressed in order to save storage space and to accelerate the transmission over transmission channels with limited bandwidth. However, monitoring the use of the music pieces distributed in transmission networks or tracking illegal copies thereof is becoming an increasing problem. While on the one hand a broad distribution of audio pieces is desirable, copyrights have to be respected. In this context, "watermarking" is a useful mechanism for tracking such illegal copies, or for incorporating copyright information, or general intellectual property, into the audio signal.

The introduction of watermarks in non-compressed multimedia data, such as B. pictures, video, audio, etc. is known. In order to introduce watermarks into compressed material, however, a fast, quality-preserving watermarking process is required.

The specialist publication "Audio Watermarking of MPEG-2-AAC Bit Streams", Christian Neubauer, Jürgen Herre, 108th AES Convention, Paris 2000, Preprint 5101 first teaches how to generate a spectral representation of an audio signal. A spread and spectrally transformed watermark signal is then added to this. A new bit stream is generated from the sum signal by quantization and Huffman coding. This so-called bit stream watermarking method is characterized by a low computational complexity, since the bit stream to be provided with a watermark does not have to be fully decoded. This method also has the advantage of high audio quality, since the quantization noise and the watermark noise can be matched to one another if the energy introduced into the audio signal by the watermark is below the psychoacoustic masking threshold lies. The method is also characterized by a high level of robustness, since the watermark cannot be removed from the re-decoded audio signal, for example by an illegal distributor of the audio signal, without impairing the audio quality.

A disadvantage of the described method, however, is the fact that the watermark may be quantized or weakened under certain circumstances by the quantization of the signal applied to the watermark. This is due to the fact that the energy of the watermark signal is sometimes in the range of the quantization interval. Furthermore, there is only limited control over the interference introduced by the watermark. B. can affect an audio quality loss.

Another watermarking method is to embed the watermark during the compression of the audio signal. This concept is described in the specialist publication "Combined Compression / Watermarking for Audio Signals", Frank Siebenhaar, Christian Neubauer and Jürgen Herre, 110th AES Convention, May 12-15, 2001, Amsterdam, Preprint 5344. First, an uncompressed audio signal is fed to a psychoacoustic model in order to determine the masking threshold. The audio signal is then transformed into the frequency domain. The spread, spectrally represented watermark signal is weighted based on the masking threshold in the frequency domain and added to the spectrum of the input audio signal. The parameters for the quantization are determined on the basis of the masking threshold, whereupon the watermarked signal is quantized and encoded. This method is also characterized by a low level of computational complexity, since certain operations, such as e.g. B. the calculation of the masking model and the conversion of the audio signal into a spectral representation only have to be carried out once. The method usually also provides one good audio quality, since quantization noise and watermark noise can be coordinated.

However, a disadvantage of this method is also the fact that the watermark may be quantized or weakened under certain circumstances by the quantization of the signal applied to the watermark. This is again due to the fact that the energy of the watermark signal is sometimes in the range of the quantization interval. Furthermore, there is only limited control over the interference introduced by the watermark. B. can affect an audio quality loss.

If the spectral representation of the audio signal is considered, a large number of audio spectral values can be seen. The spread watermark signal is also characterized by a large number of spectral lines. However, so that the watermark does not lead to audible interference in the decoded audio signal, the height of the watermark spectral lines is significantly less than the height of the audio signal spectral lines. After adding the watermark spectrum to the audio spectrum, the combined spectrum is only slightly changed compared to the original spectrum. The subsequent quantization of the combined spectrum will always remove the watermark without replacement if the quantization step size is greater than the height of the watermark spectral lines that are quantized with this quantizer step size. If too many watermark spectral lines are "quantized away" by the subsequent quantization, the watermark detector can no longer extract a clear watermark.

The object of the present invention is to provide an improved concept for embedding a watermark in an audio signal which on the one hand provides good audio quality and on the other hand also ensures good watermark detectability. This object is achieved by a method for embedding a watermark in an audio signal according to claim 1 or by a device for embedding a watermark in an audio signal according to claim 16.

The present invention is based on the finding that better watermark detectability is achieved if the fact that the audio signal including the watermark is subjected to quantization is taken into account in the watermark embedding. A watermark will only be detectable if a spectral line, which represents the watermark and audio signal, falls through the watermark into a different quantization level than if no watermark is embedded. Only in this case will a watermark detector, which only receives quantized information, be able to detect a watermark. In other words, this means that if a spectral line that represents the watermark and the audio signal falls into the same quantization level as the corresponding spectral line that only represents the audio signal, the watermark embedding was in vain, since there was no energy component in the quantized signal from the watermark will be seen. The watermark has been quantized away.

According to the invention, the spectral representation of the watermark signal is therefore processed in such a way that it is ensured that the watermark signal processed by the processing step is designed in such a way that it will still be present after quantization. In order to achieve this, a predetermined watermark starting value is selected, which depends on the spectral representation of the watermark signal. Of course, the watermark must not lead to any or only a very slight interference in the audio signal. For this reason, a disturbance introduced by the predetermined watermark start value into the spectral representation of the audio signal is determined, but in which the conditions after a quantization of the spectral representation of the audio signal can be used. On the one hand, this makes it possible to see whether some of the watermark remains after quantization. On the other hand, it can be ensured that the disturbance of the watermark after quantization is as it should be. If introduced by the watermark initial value error is greater than a predetermined Storungsschwelle, the watermark initial value is changed until the system introduced by a modified watermark initial value in the spectral representation after quantization error less than or equal to the vorbe ^¬ voted fault threshold is. The resulting changed watermark starting value is then combined with the audio signal in order to obtain the watermarked audio signal in which the watermark is embedded.

An advantage of the present invention is that ratios that ultimately do not correspond to the initial ratios are no longer taken into account, namely the audio signal / watermark ratios before the quantization, but that the watermark z. B. iteratively changed until a desired watermark "interference energy" is found. According to the invention, the conditions after the quantizer are now taken into account, i. H. the ratios that are relevant for the audio signal decoder and for the watermark extractor.

Although the watermark energy has usually been set in the prior art in such a way that the watermark energy is less than or equal to the psychoacoustic masking threshold, the unpredictability of what happens to the watermark signal during quantization still remains. As has been explained, on the one hand the case could occur that the watermark is quantized away, which means that no watermark or only a very weak watermark could be extracted in the decoded signal. On the other hand, the case could also arise that, although the watermark What has been important is that it is below the masking threshold, yet noise has been introduced that was audible in the decoded signal.

According to the invention, a precise control is now achieved due to the processing of the watermark on the basis of the conditions after the quantization. This control has the advantage that not only can it be ensured on the one hand that the watermark leads to no or only minimally audible interference, but that at the same time sufficient watermark detectability is ensured. On the other hand, the method according to the invention has the advantage that for cases in which good detectability is particularly important, certain - tolerable - disturbances are deliberately introduced into the audio signal in favor of a higher watermark detectability, while in other cases in which the watermarks -Detectability does not have to be ensured at all times under all circumstances, compromises with regard to watermark detectability can be made in order to meet the highest audio quality requirements.

In the preferred embodiment of the present invention, the watermark signal is added to the audio signal prior to quantization to obtain a combined signal. The combined signal is then quantized and again inversely quantized and compared with the original audio signal. The comparison determines whether the interference introduced by the watermark is tolerable. If it is determined that the disturbance is not tolerable, the spectrum of the watermark signal is weighted iteratively using certain strategies in order then to carry out quantization and inverse quantization again until it is determined that the disturbance is now tolerable. The watermark spectrum obtained by this processing is then added to the original audio spectrum. The added or combined signal is then quantized, entropy-encoded and provided with side information to obtain an audio bit stream in which the watermark is present.

In another embodiment of the present invention, the original audio signal is quantized. A quantized watermark is added to the audio signal to obtain the combined signal. The combined signal is then no longer quantized again, as in the first exemplary embodiment, but is directly entropy-coded. The "quantized" watermark signal added to the quantized audio signal is set in such a way that on the one hand the requirement for tolerable interference is met and on the other hand a desired watermark detectability is achieved.

Regardless of whether the combined signal is still being quantized or whether the combined signal is already in quantized form, according to the present invention precise control of the interference introduced by the watermark is achieved in that on the signal.

Preferred embodiments of the present invention are explained in detail below with reference to the accompanying drawings. Show it:

1 shows a block diagram of an inventive device for embedding a watermark in an audio signal;

2 shows a block diagram of a device according to the invention for introducing a watermark into an audio signal according to a first exemplary embodiment;

3 shows an inventive device for embedding a watermark in an audio signal according to a second exemplary embodiment; and 4a to 4d a schematic explanation of the line selection algorithm in the second exemplary embodiment of the present invention.

The device according to the invention shown in FIG. 1 comprises an audio input 10 and a watermark input 12. Both the audio signal at the audio input 10 and the watermark signal at the watermark input 12 are converted into a spectral representation by means of a device 14 or 16. The spectral representation of the audio signal comprises audio spectral values, while the spectral representation of the watermark signal has watermark spectral values. In a device 18 for combining, the audio spectral values are combined with changed watermark spectral values in order to obtain the combined audio signal in an output 20, in which the watermark is embedded. According to the invention, a device 22 for processing the spectral representation of the watermark signal is provided depending on a psychoacoustic masking threshold supplied via an input 24. The spectral representation of the watermark signal is processed depending on the psychoacoustic masking threshold obtained via the input 24 in order to obtain a processed watermark signal so that a disturbance introduced into the audio signal by the processed watermark signal is below a predetermined disturbance threshold which is different from that psychoacoustic masking threshold depends.

For this purpose, the device 22 for processing the spectral representation of the watermark signal comprises a device 26 for selecting a predetermined watermark starting value, which depends on the spectral representation of the watermark signal. In a device 28, a disturbance introduced by the predetermined watermark start value into the spectral representation of the audio signal after quantization of the spectral representation of the audio signal is detected. telt. For this purpose, quantization information is supplied by a device 30 for supplying quantization information. The device 30 provides quantization information which depends on the original audio signal, that is to say the audio signal without a watermark.

A device 32 examines whether the ascertained disturbance is greater than the predetermined disturbance threshold. If not, i. H. if the disturbance is acceptable, the watermark start value is fed directly to the device 18 for combining. If this is the case, however. H. if the introduced interference is too great or different than desired, a device 34 for changing the watermark start value is activated until the interference introduced by a changed watermark start value in the spectral representation of the audio signal after quantization is less than or equal to the predetermined one Interference threshold is. For this purpose, the loop sketched in the processing device 22 may have to be iterated several times in order to at some point obtain a changed watermark starting value at the output of the device 32, which is used as a processed watermark signal and fed to the combining device 18 in order to to receive the audio signal at the output 20 in which the watermark is embedded.

In a preferred exemplary embodiment of the present invention, which is shown in FIG. 2, the combination is carried out by means of an addition 18 before the quantization. The device 28 for determining the initial value introduced into the audio signal by the block watermark weighting 26 is determined by first quantizing and inversely quantizing the combined signal in a quantizer / inverse quantizer device 28a. The disturbance introduced by the watermark is then calculated in a device 28b, for example by forming the difference and squaring the difference values, and then in the device 32 with the psy- choacoustic masking threshold 24 compared. If the disturbance is too great, the device 34, which is labeled "weight control" in FIG. 2, is actuated in order to supply changed weighting factors to the block 26, in order then to change the weighted spectrum of watermarks in the device 18 with the original audio signal in to combine spectral representation and to go through the iteration loop again.

In the exemplary embodiment shown in FIG. 2, it is preferred to use the watermark spectrum with a watermark spectrum weighted equally for all spectral lines as the initial watermark value. The weighting factor for each spectral line is therefore equal to a constant for all spectral lines, which is chosen such that the watermark energy lies above the masking threshold. Then the watermark energy is gradually reduced in order to then "iterate" the energy of the watermark below the masking threshold.

So if it is first determined by the device 32 that the disturbance is too large, the device 34 is designed to control the weighting factors in order to reduce all weighting factors, e.g. B. cut in half. If the disturbance is then still too large, all current weighting factors could be halved again in a next iteration step, etc. This can continue until the device 32 determines that the disturbance is now OK.

After the combination of audio signal and processed watermark signal takes place in the spectral range, that is, not in the quantization range, but before the quantization, a quantization must be carried out. For this purpose, it is advisable to use the quantizer section of the device 28a in order to output the output values of the quantizer section as an audio signal including the embedded watermark. The disturbance caused by the watermark after quantization is thus determined by means of an analysis-synthesis iteration, as shown in FIG. 2. On the one hand, this ensures that watermark energy remains in the signal even after quantization. On the other hand, the interference actually introduced can be determined, which is advantageous for achieving high audio quality. The spectrally represented watermark signal is thus spectrally weighted with the current weighting factors provided by block 34 by means of a weighting filter bank, which can be contained in block 26, as has been explained with reference to FIG. 2. The resulting signal is added to the original audio signal. As has been stated, the combined signal at the output of device 18 is quantized and inversely quantized and results in the signal present at the output of device 28a which is fed into device 28b in the same way as the original audio signal. The device 28b now compares the original signal with the quantized and again inversely quantized signal and uses this to determine the quantization error signal which is supplied to the device 32. On the basis of the device 32, the weighting control in block 34 is activated in order to determine new, better weighting factors. For this purpose, the masking threshold determined by the masking model is available, which indicates how much interference in the signal is "allowed" at a certain point in the signal spectrum. If the weight control block 34 has determined optimal weighting factors with regard to the desired audio signal interference and the desired watermark detectability, ie watermark energy, the method terminates. The quantized spectral values of the combination signal last determined by block 28a are then passed on to the bitstream multiplexer as a result, in order to be formed there together with the side information into an audio bitstream. 3 is discussed below in order to illustrate a device for embedding a watermark in an audio signal according to a second exemplary embodiment of the present invention. In contrast to the first exemplary embodiment shown in FIG. 2, in which an unquantized audio signal is combined with an unquantized watermark signal, this combination 18 in FIG. 3 takes place in the "quantization range", ie it becomes a quantized audio signal with a combined quantized watermark. This can be achieved either by using a quantizer 42 to calculate the quantizer stages by quantizing the original audio signal, or by extracting the quantizer stages from a coded audio signal. In response to the quantization levels provided by means 42, means 40a for calculating the quantized audio signal minus a predetermined number of n quantization levels and means 40b for calculating the quantized audio signal plus a predetermined number of n quantization levels are operated.

In contrast to the exemplary embodiment shown in FIG. 2, in which a quantization calculation and an inverse quantization calculation had to be carried out by the device 28a for each combined audio signal, specifically within the iteration loop, this occurs in the second exemplary embodiment a shown in FIG. 3 -priori instead, ie by precalculation outside an iteration loop. For this purpose, a so-called “maximum” watermark is first calculated as a predetermined watermark starting value by means of a device 36. To calculate the predetermined maximum watermark, only the signs of the watermark spectrum are used. If the watermark spectrum has a positive sign, the corresponding spectral value of the original quantized audio signal is increased by n quantization levels, n being an integer greater than or is 1. If, on the other hand, the sign of a watermark spectral value is negative, the corresponding quantized spectral value, ie the spectral value of the audio signal at the same frequency as the spectral value of the watermark signal whose sign is currently being viewed, is reduced by n quantization levels. This results in a maximum watermark, the expression "maximum" being understood to mean that the maximum watermark signal has an effect on each spectral line of the original audio signal after quantization. Although this case is desirable in terms of very good watermark detectability, experience has shown that it may introduce too much interference into the audio signal. In order to reduce the disturbance to an acceptable level, an acceptable level being, for example, the psychoacoustic masking threshold, a device 38 that implements a line selection algorithm is provided. The device 38 determines the interference introduced into the audio signal by the maximum watermark provided by the device 36. If the disturbance is greater than the predetermined disturbance threshold, the device 38 changes the "maximum" watermark by selecting individual lines until the disturbance introduced by the watermark is less than or equal to the predetermined disturbance threshold. If this condition is fulfilled, the watermark, which is already in quantized form, and the quantized original audio signal are fed to the adder 18 in order to obtain the quantized watermarked audio signal on the output side.

The function and mode of operation of the devices 36 and 38 will be discussed below with reference to FIGS. 4a to 4d. 4a shows, by way of example, a quantized audio signal which, because of the clarity of the illustration, only shows three spectral values 50a-50c. Typically, depending on the selected window length and transformation, an audio spectrum has e.g. B. 1024 spectral values. The The number of non-zero quantized spectral values depends on how many audio spectral values have been quantized to zero. Of course, the quantized audio spectral values have different heights in the real case. 4b shows an audio spectrum with plus or minus n quantization levels (depending on the sign of the watermark spectral values). The spectral component of the watermark corresponding to the audio spectral value 50a of FIG. 4a has a negative sign for the example shown in FIG. 4b. The spectral component of the watermark, which corresponds to the audio spectral value 50b of FIG. 4a, has a positive sign in the example shown in FIG. 4b, while the third spectral component of the watermark again had a negative sign. The amount of the watermark spectral components is initially irrelevant, since it is assumed that watermark detection is already possible if the quantized audio spectral values 50a-50c are changed by the watermark. The maximum watermark, which is determined by the device 36 from FIG. 3, is shown in FIG. 4c for the case shown in FIG. 4b. It has a spectrum that is characterized in that each quantized original audio spectral value is changed by a quantization level, either enlarged if the watermark has a positive sign, or decreased if the watermark had a negative sign.

In the example shown in FIG. 4b, the amount of a watermark spectral line could be taken into account in such a way that not only is incremented or decremented by one quantization level, but that is incremented or decremented by several quantization levels if the amount of Watermark spectral line is correspondingly large.

The function of the device 38 from FIG. 3 will now be described with reference to FIG. 4d. Provides the facility 38 If the left quantized audio spectral component 50a is reduced by a quantization level, as is represented by the spectral component 50a ', the situation is such that the situation is such that the interference introduced by the watermark is too large, as is represented by the spectral component 50a', this becomes the spectral component not selected by the device 38, which is so noticeable in the changed watermark spectral values after the line selection that the changed watermark has a spectral line of 0 at this point. In the middle and right spectral components of the quantized audio signal, on the other hand, it was found that the interference introduced by spectral lines 50b 'and 50c' was in order, so that so much watermark energy can be added to the quantized audio spectral values at these points that these can be increased by a quantization level (50b ') or decreased by a quantization level (50c').

From this consideration it becomes clear that by precalculating the quantization levels by means 40a and 40b, the step of quantization and inverse quantization, i. H. the device 28a of FIG. 2 can be omitted, since the size of the disturbance can be pre-calculated a priori by changing the quantization index. It can also be seen from Fig. 3 that the device 26, d. H. the weighting of the watermark spectral lines has been eliminated.

The quantized audio spectral values are now based on the watermark signal, ie using the sign of the watermark signal by z. B. plus or minus changed a quantization level. This procedure has advantages in that computing time can be saved since the quantization and inverse quantization (device 28a of FIG. 2) and the weighting of the watermark (device 26 of FIG. 2) can be omitted without replacement. The maximum watermark (FIG. 4c) is determined line by line on the basis of the audio spectra already precalculated, ie the original spectrum and the original spectrum minus n quantization levels or the original spectrum plus n quantization levels. This results as the difference between the original spectrum (FIG. 4a) and the audio spectrum changed by a number of n quantization levels (FIG. 4b), the difference having the same sign as the unweighted watermark.

The line selection algorithm, which is executed in the device 38, takes into account the amount of the unweighted watermark spectral lines, the masking threshold 24 and possibly a bit savings bank function 44 of the audio encoder.

In order to ensure both good audio quality and good watermark detectability, it is preferred to select the lines of the maximum watermark in such a way that the watermark spreading band signal is embedded in a broadband manner, i. H. that as many lines of the quantized audio signal as possible are changed. Furthermore, the masking threshold or, if a threshold deviating from the masking threshold is used, this predetermined disturbance threshold should not be violated. Finally, the structure of the watermark should be changed as little as possible within a frequency band.

All other lines of the maximum watermark are not taken into account. This means that after the addition of the watermark, the quantized audio spectral values of the selected lines are changed by plus or minus n quantization levels, while the quantized audio spectral values of the non-selected watermark lines are adopted unchanged. The quantized watermarked audio signal at the output 20 of the device shown in FIG. 3 now has to be entropy-coded.

Depending on the audio coding method used, in which the concept according to the invention is integrated, there is a bit savings bank function which can provide additional bits to later signal blocks, as has been carried out. The line selection strategy is preferably adapted to the fill level of the bit savings bank, so as to allow, for example, when the bit savings bank is filled, that even quantized audio spectral values of the original audio signal that have the value 0 are watermarked, which would normally not be permitted due to the bit requirement , This can noticeably improve watermark detection.

When using the combined embedding / coding, in addition to the already quantized audio spectral values, the original values are also available after the transformation into the frequency range. The quantization of the original audio spectral values can also be seen as a kind of watermark embedding, since a certain disturbance of the audio spectrum results in both the quantization and the addition of a watermark signal. Due to its random nature, the disturbance introduced by the quantization should not be regarded as a watermark. However, if the introduced disturbance is correct with the watermark due to the quantization, the quantization noise supports the detectability of the watermark. The following cases result from this.

By quantizing an audio spectral line, the watermark is introduced with the correct sign. Here, the device 38 of FIG. 3 is preferably arranged such that, due to the fact that the quantization is already in phase with the watermark spectral value for one certain frequency a disturbance has been introduced, arranged to dispense with the introduction of another watermark disturbance. Alternatively, a quantization level could be added to further improve the detectability.

On the other hand, if an interference is introduced by the quantization of an audio spectral line, which has the opposite sign as the watermark signal, which leads to the watermark being worsened to a certain extent by the opposite quantization, the line selection strategies which have been explained above must be considered. whether the robustness of the watermark must be guaranteed for this line and thus the quantized audio spectral value must be changed in order to "reverse" the quantization noise to a certain extent, or whether the embedded watermark at this point, in view of a better audio quality. H. the quantization noise at this point is said to have a "wrong" sign.

As has already been explained, in modern coding methods the psychoacoustic masking threshold is not calculated line by line, but by scale factor band. This means that energies of individual spectral lines are not considered, but the total energies z. B. 20 spectral lines in a scale factor band. However, in a scale factor band in which many watermark spectral lines are tolerable, a few lines in the sense of good audio quality can be dispensed with without the watermark detectability suffering significantly. This functionality can also be achieved in the exemplary embodiment shown in FIG. 2 in that the weighting control 34 of FIG. 2 is designed such that the same weighting factors are not used over the frequency, but rather that different weighting factors are used for different spectral values, and that especially on weight factors of 0 occur for individual spectral lines. As a predetermined watermark starting value, it can also be advantageous in the exemplary embodiment shown in FIG. 2 to design the watermark weighting before the iteration begins so that it is derived from the psychoacoustic masking threshold.

In summary, the concept according to the invention is such that a spectrally represented watermark signal is first generated. This is weighted using weighting factors. The weighted signal is added to the original audio signal, which is available in a spectral representation. Alternatively, a change in the lines of the original audio signal, which is available in a spectral representation, is carried out on the basis of the watermark signal. The disturbance introduced after the quantization is then determined, the disturbance being determined by quantizing, inverse quantizing and forming the difference from the original, or the disturbance being pre-calculated.

Then new weighting factors are determined, using the masking threshold, using a line selection strategy, or using a line selection strategy in particular in such a way that the sign and amount of the spectral lines of the unweighted watermark are used, and that the sum of the watermark line and the original spectral line is such it is determined that this new spectral line falls within a different quantization interval than the original spectral line.

The concept according to the invention is advantageous in that it can be used both for bitstream watermarking methods and for methods which carry out audio coding and watermark embedding in one step. Another advantage of the concept according to the invention is that full control over the introduced disturbance can be achieved. This makes it possible to specifically set the method in favor of optimal watermark detection or optimal audio quality.

Another advantage of the concept according to the invention is full control over the frequency distribution of the watermark spreading band signal into the audio signal.

Claims

claims 1. A method for embedding a watermark in an audio signal, comprising the following steps: Providing (14) a spectral representation of the audio signal, the spectral representation of the audio signal having a plurality of audio spectral values; Providing (16) a spectral representation of the watermark signal, the spectral representation of the watermark signal having a plurality of watermark spectral values; Processing (22) the spectral representation of the watermark signal depending on a psychoacoustic masking threshold (24) of the audio signal in order to obtain a processed watermark signal so that a disturbance inserted by the processed watermark signal into the audio signal is below a predetermined interference threshold which is different from that psychoacoustic masking threshold depends; and Combining (18) the processed watermark signal and the audio signal to obtain a watermarked audio signal in which the watermark is embedded, the step of processing (22) comprising the following substeps: Selecting (26) a predetermined watermark starting value that depends on the spectral representation of the watermark signal; Determination (28) of a start value determined by the predetermined watermark in the spectral representation of the diosignal after a quantization of the spectral representation of the audio signal introduced interference; and if the interference introduced by the watermark spectral value is greater than the predetermined disturbance threshold (32), change (34) the watermark start value until the watermark start value is incorporated into the spectral representation of the audio signal after quantization of the audio signal - led disturbance is less than or equal to the predetermined disturbance threshold, and using the changed watermark start value as the processed watermark signal. Method according to claim 1, in which in the sub-step of dialing 26 watermark spectral values are weighted with start weighting factors; in which, in the step of determining (28), the watermark spectral values weighted with the start weighting factors are added to the audio spectral values in order to obtain addition spectral values, in which the addition spectral values are quantized and then inversely quantized (28a) in order to obtain inversely quantized addition spectral values; in which the inversely quantized addition spectral values are compared with the audio spectral values (28b) in order to determine whether the interference contained in the addition spectral values is below the predetermined interference threshold (32); and in which the start weighting factors are changed in the partial step of changing (34). 3. The method of claim 2, wherein the start weighting factors are the same for all watermark spectral values and have an amount which is selected so that the energy of the watermark lies above the psychoacoustic masking threshold. 4. The method according to claim 2, in which the starting weighting factors are obtained by weighting the watermark spectral values with the psychoacoustic masking threshold, so that the energy of the watermark spectral values weighted with the psychoacoustic masking threshold is approximated to the psychoacoustic masking threshold and in particular less than or equal to the psychoacoustic Masking threshold is. 5. The method according to claim 3 or 4, wherein the start weighting factors in the partial step of changing (34) are reduced per one iteration step. 6. The method according to any one of claims 2 to 5, wherein the step of combining (18) comprises combining the spectral values of the audio signal and the spectral values of the processed watermark signal and then the step of quantizing the watermarked audio signal using quantization levels were determined by quantizing the audio spectral values without a watermark signal using the psychoacoustic masking threshold in order to obtain a quantized audio signal with a watermark. 7. The method according to claim 1, in which the sub-step of selecting (26) a watermark starting value has the following sub-steps: Determining (42) quantization levels for the audio spectral values without the watermark signal using the psychoacoustic masking threshold (24); Quantizing (42) the audio spectral values using the determined quantization levels to obtain quantized audio spectral values; Extracting signs of the watermark spectral values; Computing (36) quantized spectral values of the watermark seed so that a quantized spectral value of the watermark seed is equal to a number of quantization levels if the sign of the corresponding spectral value of the watermark signal is positive and that a quantized spectral value of the watermark seed is equal to that Negatives of a number of quantization levels if the sign of the corresponding spectral value of the watermark signal is negative; and wherein the step (34) of changing comprises the step of dividing the number of quantization levels and / or the step of selecting (38) spectral lines of the watermark start value as the changed watermark start value. 8. The method as claimed in claim 7, in which, for quantized spectral values of the audio signal which are equal to 0, no spectral values of the watermark starting value are selected as the changed watermark starting value. 9. The method as claimed in claim 7, in which a bit savings bank function (44) is present, and in which, depending on a filling of the bit savings bank, for quantized spectral values of the audio signal which are equal to 0, ral values of the watermark start value can be selected as a changed watermark start value. 10. The method according to any one of claims 1 to 9, wherein the step of changing (34) is carried out so that the largest possible number of changed watermark spectral values is not equal to 0. 11. The method according to any one of claims 1 to 10, in which the step of changing (34) is carried out in such a way that the course of the changed watermark starting value over frequency corresponds as closely as possible to the spectral course of the watermark signal. 12. The method according to any one of claims 7 to 11, in which quantized audio spectral values are added to selected watermark spectral values in order to obtain a quantized watermarked audio signal. 13. The method according to any one of the preceding claims, in which the sub-step of changing (34) is terminated when the disturbance threshold is reached or undershot, and when the number of changed watermark spectral values is above a predetermined threshold. 14. The method of claim 13, wherein the predetermined energy threshold is defined in that a predetermined number of audio spectral values of a signal comprising the audio spectral values and the changed watermark spectral values are changed by at least one quantization level compared to the quantized spectral values of the audio signal alone. 15. The method according to any one of the preceding claims, in which the psychoacoustic masking threshold (24) has a value per one scale factor band, and in which the step of processing (22) is carried out on a scale factor band basis. 16. Device for embedding a watermark in an audio signal, having the following features: a device for providing (14) a spectral representation of the audio signal, the spectral representation of the audio signal having a plurality of audio spectral values; means for providing (16) a spectral representation of the watermark signal, the spectral representation of the watermark signal having a plurality of watermark spectral values; a device for processing (22) the spectral representation of the watermark signal depending on a psychoacoustic masking threshold (24) of the audio signal in order to obtain a processed watermark signal so that a disturbance inserted into the audio signal by the processed watermark signal is below a predetermined disturbance threshold, which depends on the psychoacoustic masking threshold; and means for combining (18) the processed watermark signal and the audio signal to obtain a watermarked audio signal in which the watermark is embedded, the processing device (22) having the following features: means for selecting (26) a predetermined initial watermark value which depends on the spectral representation of the watermark signal; means for determining (28) a disturbance introduced by the predetermined watermark start value into the spectral representation of the audio signal after quantization of the spectral representation of the audio signal; means (32) for determining whether the disturbance introduced by the initial watermark value is greater than the predetermined disturbance threshold; and means (34) for changing the watermark spectral values until the interference introduced by a changed watermark start value into the spectral representation of the audio signal after quantization is less than or equal to the predetermined interference threshold, and for using the changed watermark spectral values as the processed one watermark signal.